1. Field of the Invention
The present invention relates to an apparatus for reproducing received data, for example, an apparatus for reproducing received data in a network termination apparatus.
In an integrated service digital network (ISDN) which provides audio, visual, data, and other forms of information communication services integrated using digital signals, the user-network interface between the users in the ISDN and the network is specified by the International Telegraph and Telephone Consultative Committee (CCITT) in its I-series recommendations. This is usually known therefore as an "I-interface".
The characteristic feature of an I-interface is the use of a so-called "bus connection" for the connection between the network termination apparatus of the master station, that is, the apparatus for terminating the subscriber lines connected to the network, and the plurality of units of subscriber terminal equipment forming the plurality of slave stations, that is, the group of terminal equipment connected to the subscriber lines.
Since such a bus connection is used, in upstream communication from the subscriber terminal equipment (slave stations) to the network termination apparatus (master station), the network termination apparatus (master station) receives signals having a variety of pulse phases and signals having a variety of amplitudes at its input end. The reason is that the distance between each of the plurality of units of subscriber terminal equipment (slave stations) connected in a multidrop mode with respect to the bus and the network termination apparatus (master station) differs with each subscriber line (each slave station).
Therefore, a particular technique for reproduction of the received data is required for the network termination apparatus (master station) to receive the signals sent from the different units of subscriber terminal equipment (slave stations) and reproduce them as received data. The present invention relates to this technology for reproduction of received data.
2. Description of the Related Art
As will be explained in detail later with reference to the drawings, optimal reproduction of received data can be realized by providing in the network termination apparatus a plurality of types of timing clocks having different phases corresponding to the variety of units of subscriber terminal equipment and to suitably switch among these timing clocks.
If this is done, however, it becomes difficult to select from among the variety of timing clocks without error. Therefore, an automatic decision/switching circuit which automatically performs a series of operations for deciding on the optimum timing clock among the plurality of clocks and switching to that optimal timing clock becomes necessary. An apparatus for reproducing received data actually incorporating such an automatic decision/switching circuit, however, becomes complicated, large in size, and high in cost, so is not practical.
In view of this situation, design has been made of an apparatus for reproducing received data (FIG. 4), explained in detail later, which enables switching between two types of clocks--an adaptive clock (CK.sub.a) and a fixed clock (CK.sub.f).
This conventional apparatus for reproducing received data, however, requires means for generating the adaptive clock (CK.sub.a) and the fixed clock (CK.sub.f) and a decision and switching means for selecting among these clocks, so also suffers from the problem of a more complicated construction.